Interior doors play an integral role in controlling the movement of sound through a house. They are typically the thinnest barrier in a wall and, as such, they don’t benefit from the thicker—sometimes insulated—construction of walls.
A typical interior door has a hollow core—inner cardboard honeycomb cores surrounded by a softwood frame. The door’s surfaces are faced with very thin wood veneers. Between the thin surfaces and the air-filled core, there isn’t much there to block the movement of sound.
An exterior door or an interior door with a solid core will do a much better job because of their density. Many types are available, from expensive hardwood to more affordable Medium Density Fiberboard (MDF). Though most interior doors are 1 3/8-inch thick, exterior doors are typically 1 3/4-inch thick. The thicker the door, the better it will perform at blocking sound.
Of course, it doesn’t matter how the door is built if it’s open, right? Similarly, if there are gaps around the edges or between the bottom of the door and the floor, sound will sneak around it. So the door should fit the jamb tightly, and weatherstripping should seal it. Rubber bulb weatherstripping gaskets and a rubber loop door-bottom weatherstripping sweep do a good job of sealing around the perimeter.
Sound blocking materials are rated by an STC (Sound Transmission Class) rating. If you were to replace a hollow-core interior door that has an STC of less than 20 with a solid-core door that is properly weatherstripped, what would be the result? According to the National Wood Window and Door Association, “If you did all of this, you could probably end up with an STC rating of 34 to 36.” For more about STC ratings, please see Soundproofing Walls & Ceilings.
When planning for new doors and windows, also consider where sound travels. If possible, stagger doors along a hallway and arrange their swing so that they don’t deflect sound into adjoining rooms. Choose hinged doors; avoid sliding, bi-fold, and pocket doors that not only make noise themselves but also don’t seal as well as the swinging type.
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